1. Field of the Disclosure
The present disclosure relates generally to an image forming apparatus and, more particularly, to systems and devices for transferring toner in an electrophotographic imaging system.
2. Description of the Related Art
Transfer process, whereby toner is moved from a donating medium to an accepting medium, is a core process in an electrophotographic printing process. The process starts when a photosensitive member, such as a photoconductor, is charged and then selectively discharged to create a charge image. The charge image is developed by a developer roll covered with charged toner of uniform thickness. This developed image then travels to what is referred to as “first transfer” in the case of a two-step transfer system, or the only transfer process in the case of direct-to-paper systems.
Transfer robustness is frequently measured as the amount of voltage between the lowest voltage at which acceptable transfer occurs due to a sufficient electric field having been established to move toner, and the highest voltage at which acceptable printing occurs before Paschen breakdown, i.e., the voltage at which the dielectric properties of the materials in the transfer nip begin to break down, causes undesirable print artifacts. The larger the difference between the lowest and highest voltages, the more tolerance exists for part-to-part variation while still yielding relatively good quality prints. The lower end of the transfer operating window is typically determined by how well the electric field, measured in volts/meter, can be established, and by how much electric field is then required to overcome the forces of adhesion between the toner and the donating medium (photoconductor or belt). The upper end of the transfer operating window is the point at which the electric field established to transfer the toner exceeds the breakdown strength of an air gap or dielectric layer, allowing a discharge event to occur.
In traditional first transfer systems, the developed toner enters a transfer station or nip area between a photoconductor roll and a transfer roll. The media to which the developed toner image is to be transferred, either an intermediate transfer member (ITM) for a two-step transfer system or a transport belt supporting paper for a direct-to-paper system, is positioned between these two rolls. Time, pressure and electric fields all influence the quality of the transfer process. A voltage is applied to the transfer roll to create a field to pull charged toner off the photoconductor roll onto the desired medium.
Relatedly, in traditional two-step transfer systems, the ITM, now carrying the charged toner, travels to a second transfer station or nip area, similar in some ways to the first transfer nip. The toner is again brought into contact with the toner receiving medium in the second transfer nip formed by a number of rolls. Typically a conductive backup roll and a resistive transfer roll together form the two primary sides of the second transfer nip. As with the first transfer, time, pressure and applied fields play significant roles in ensuring high efficiency transfer.
The above traditional roller-based transfer configurations have served transfer systems well. However, roller hardware has several deficiencies that have become more evident as process speeds are increased and support for a broader set of operating environments is extended. To illustrate these deficiencies, FIGS. 1-2 are depicted which are based on outputs from finite element models. It should be noted that the example configurations of FIGS. 1-2 are illustrated for demonstration purposes only.
FIG. 1A illustrates an example of a roller-based transfer configuration having a transfer roller 10A with a 0 mm offset arrangement relative to a photoconductive drum 15A (or a nip 20A formed by the photoconductive drum 15 A and an ITM 25A), FIG. 1B illustrates an example of another roller transfer configuration having a transfer roller 10B with a 1.5 mm offset arrangement downstream from a photoconductive drum 15B (or a nip 20B formed between the photoconductive drum 15B and an ITM 25B), while FIG. 2 is a diagram illustrating graphs 17A, 17B of electric field magnitudes in the air gaps at the nip regions as a function of roller placement relative to nip 20 (at 0 mm) for each of the roller configurations of FIGS. 1A and 1B, respectively. FIG. 2 further shows a curve 18 corresponding to the air gap between the ITM 25 and photoconductive drum 15. In these examples, process direction is from left to right such that photoconductive drums 15 and transfer rollers 10 rotate counter-clockwise and clockwise, respectively.
For the configuration shown in FIG. 1A, when a corresponding bias voltage is applied to transfer roller 10A, relatively high electric field values may develop on the underside of ITM 25A post nip (illustrated in FIG. 2, peak electric field 30A of graph 17A occurring on the underside of ITM 25A is located after 0 mm nip position), due in part to displacement currents created by capacitive coupling effects between transfer roller 10A and ITM 25A. These displacement currents are created as the separation distance between the surface and the transfer roller surface changes. In particular, as the transfer roller surface approaches the nip 20A, voltage differential decreases with separation distance and reduces the electric field, and as the transfer roller surface exits the nip, voltage differential increases and intensifies electric field post nip. This effect will also be dependent upon how quickly the air gaps open and close (i.e., depending on process speed and roller geometry) and how quickly the roller may respond to the changing electric field (i.e., depending on transfer roller resistivity or moisture content). This peak electric field 30A (FIG. 2) located post nip and on the underside of ITM 25A may cause a “first transfer over transfer” failure which results from breakdown in the air gap between the transfer roller 10A and ITM 25A prior to the point at which an electric field sufficient to transfer toner from the photoconductive drum 15A to ITM 10A is built. This type of failure causes discharge events which may disrupt the electric field between the photoconductive drum and ITM 25A, and may lead to additional breakdown events or disturb the toner on ITM 25A, resulting in poor transfer.
For the configuration shown in FIG. 1B, when a corresponding bias voltage is applied to the transfer roller 10B, a peak electric field 30B (FIG. 2) may develop on the top side of ITM 25B a greater distance from the 0 mm nip position due at least in part to the diffuse nature of the roller and capacitive coupling effects. The consequence of this peak field location post nip is a “negative ghosting” failure which results from breakdown in the air gap between ITM 25A and photoconductive drum 15B. This breakdown event deposits charges on the surface of the photoconductive drum and causes additional toner to be deposited on the photoconductive drum surface during subsequent development steps, resulting in locally darker print in future images.
In both example cases, the electric fields are asymmetrically skewed post nip because of capacitive coupling effects, thereby making it difficult to predict the peak field location as process speed changes. Additionally, the peak field 30B location for the 1.5 mm offset roller of FIG. 1B is positioned further downstream from the nip 20 relative to the peak field 30A for the 0 mm arrangement of FIG. 1A, further demonstrating the sensitivity of the roller system to mechanical tolerances. Thus, part variation may drastically impact where the peak electrical field is established. Due to the diffuse nature of a roller system, high strength electric fields are also developed wherever large voltage differential exists across an air gap, such as at distances far removed from the nip 20 across air gaps in non-functional regions surrounding the nip 20 and on the underside of the ITM 25. For example, in FIG. 2, field values greater than 1×107 V/m are sustained for a distance of approximately 1 mm around the nip 20A for the configuration shown in FIG. 1A, and for a distance of approximately 2.5 mm from the nip 20B for the configuration shown in FIG. 1B. Sustaining high strength fields for longer than is necessary may provide the system with a greater opportunity to discharge in an unintended fashion.
Thus, the field shape generated by a roller in a roller-based transfer system is diffused which generally makes it difficult to accurately place the peak field location relative to the nip. Additionally, high strength electric fields are developed across air gaps in non-functional regions surrounding the nip and on the underside of the belt. Furthermore, electric fields are also distorted by capacitive coupling effects and displacement currents may contribute to discharge events post nip which may further limit the upper end of the transfer window.
Based upon the foregoing, there is a need for an improved transfer system in an electrophotographic imaging device.